The presently disclosed embodiments are directed to imaging members used in electrostatography. More particularly, the embodiments pertain to electrophotographic imaging members which have improved formulations to effect surface contact friction reduction for wear resistant enhancement and suppress copy printout defect caused by chemical attack through the addition of a protective overcoat layer prepared to comprise of: (1) a polymer blend of a low surface energy copolymer and a chemically resistive copolymer, (2) a chemically resistive copolymer and a slip agent, and (3) a chemically resistive copolymer and a dispersion of a low surface energy Polyhedral Oligomeric Silsesquioxane (POSS) nanoparticles. This disclosure relates to all types of electrophotographic imaging members used in electrophotography.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Flexible electrostatographic imaging members are well known in the art. Typical electrostatographic imaging members include, for example: (1) electrophotographic imaging members (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging members for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. All the electrostatographic imaging members are prepared in either flexible belt form or rigid drum configuration and could either be a negatively charged or positively charged design.
For a typical flexible electrophotographic imaging member belt used in a negatively charged imaging system, the imaging member belt comprises a charge transport layer, a charge generating layer, and optional layers on one side of a supporting substrate layer and does also include an anticurl back coating on the opposite side of the substrate to imaging member flatness. In this flexible electrophotographic imaging member, where the charge generating layer is sandwiched between the top outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer should be capable of generating electron hole pair when exposed imagewise and inject only the holes through the charge transport layer. In the alternate case where the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer which also provided with an anticurl back coating to render imaging member flatness.
A typical flexible electrographic imaging member belt may, however, have a more simple material structure and include a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Alternatively, the electrostatographic imaging members can also be a rigid member, such as those utilizing a rigid substrate support drum. For these drum imaging members, having a thick and rigid cylindrical supporting substrate bearing the imaging layer(s), no application of an anticurl back coating layer is needed.
All the flexible electrostatographic imaging members may be seamless or seamed belts. Seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam.
Although the scope of the present embodiments covers the preparation of all types of electrostatographic imaging members in flexible belt design or rigid drum configuration, but for reasons of simplicity, the discussion hereinafter will focus and be represented only by flexible electrophotographic imaging member belts of negatively charged design.
Electrophotographic flexible belt imaging members may include a photoconductive layer including a single layer or composite layers. The flexible belt electrophotographic imaging members may be seamless or seamed belts. Seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. The typical negatively charged electrophotographic imaging member belts include a top outermost exposed charge transport layer directly over a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer to render flatness. By comparison, a typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate. Since typical negatively-charged flexible electrophotographic imaging members exhibit undesirable upward imaging member curling after completion of coating the top outermost exposed charge transport layer, an anticurl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is key to provide the appropriate imaging member with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a negatively-charged photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer.
Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer is sandwiched between the outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the surface of the top outermost exposed charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the top exposed charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member web like photoreceptor, the conductive layer may be a thin coating of metal on a flexible substrate support layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anticurl back coating layer on the side of the conductive layer/substrate support opposite the side carrying the blocking layer, adhesive layer, charge generating layer, charge transport layer and other layers.
For typically negatively-charged imaging member belts, such as flexible photoreceptor belt designs, they are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer is the last and thickest layer to be coated to become the outermost exposed layer and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the charge transport layer coating through drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the charge transport layer in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer exhibits a larger dimensional shrinkage than that of the substrate support as the imaging member web stock (after through elevated temperature heating/drying process) as it cools down to ambient room temperature. This dimensional contraction mis-match results in tension strain built-up in the charge transport layer, at this instant, is pulling the imaging member web stock upward to exhibit curling. If unrestrained at this point, the imaging member web stock (using 3½ mil-thick polyethylene terephthalate substrate and a 29 micrometer charge transport layer coated thickness) will spontaneously curl upwardly into a 1.5-inch roll. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
One layer of the flexible imaging member belt, for example, the top outermost exposed charge transport layer of a negatively charge imaging member, is constantly subjected to and suffer from the machine operational conditions, such as exposure to high surface friction interactions and extensive cycling. Such harsh conditions lead to wearing away and susceptibility of surface scratching of the charge transport layer which otherwise adversely affect machine performance. Another imaging member functional problem associated with the charge transport layer is its propensity to give rise to early development of surface filming due its high surface energy; charge transport layer surface filming is undesirable because it does pre-maturely cause degradation of copy printout quality. Moreover, the outermost exposed charge transport layer is also been found to exhibit early onset of surface cracking, as consequence of repetition of bending stress belt cyclic fatiguing, airborne chemical species exposure, and direct solvent contact, under a normal machine belt functioning condition. charge transport layer cracking is a serious mechanical failure since the cracks do manifest themselves into defects in print-out copies. All these imaging member layers failures are major issues that remain to be resolved as they pre-maturely cut short the functional life of an imaging member and prevent it from reaching the belt life target. Early imaging member functional failure thereby requires frequent costly replacement in the field.
A number of current flexible electrophotographic imaging member belts are multilayered photoreceptor belts that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional anticurl back coating at the opposite side of the substrate support to render flatness. In such an imaging member belt design, the charge transport layer is therefore the top outermost exposed layer. In a conventional electrophotographic imaging machine design, a flexible imaging member belt is mounted over and encircled around a belt support module comprising numbers of belt support rollers, such that the top outermost exposed charge transport layer is subjected to all electrophotographic imaging subsystems mechanical actions, chemical attacks by corona species emission from charging devices, as well as environmental contaminant vapor exposure. In essence, the top exposed charge transport layer surface of the flexible imaging member belt, during normal machine electrophotographic imaging and cleaning operating conditions, is constantly under physical/mechanical/electrical/chemical interactions, such as for example, the mechanical sliding actions of cleaning blade and cleaning brush, electrical charging devices corona effluents exposure, developer components, image formation toner particles, hard carrier particles, debris and loose CaCO3 particles from receiving paper, and the like during dynamic belt cyclic motion. As a consequence of these interactions against the imaging member belt, the exposed top charge transport layer is found suffer from surface scratching, abrasion, and exacerbating wear. In some instances, the charge transport layer has been found to wear away by as much as 10 micrometers after approximately 20,000 dynamic belt imaging cycles. Excessive charge transport layer wear is a serious problem because it causes significant change in the charged field potential and adversely impacts copy printout quality. Another outcome of charge transport layer wear is the decrease of charge transport layer thickness alters the equilibrium of the balancing forces between the charge transport layer and the anti-curl back coating and impacts imaging member belt flatness. The reduction of charge transport layer thickness by wear causes the imaging member belt to curl downward at both edges. Edge curling in the belt is an important issue because it changes the distance between the belt surface and the charging device(s), causing non-uniform surface charging density which manifests itself as a “smile” print defect on paper copies. Such a print defect is characterized by lower intensity of print-images at the locations over both belt edges. Moreover, the susceptibility of the charge transport layer surface to scratches (caused by interaction against developer carrier beads and the hard CaCO3 particles and debris from paper) has also been identified as a major imaging member belt functional failure since these scratches do manifest themselves as print defects in paper copies. More over, development of charge transport layer surface film due to the result of high surface energy has also frequently been found to be problematic, since the film formed on the charge transport layer surface does affect toner image formation which thereby causes copy print out quality degradation.
Another emerging problem recently found is associated with chemical contaminants exposure/interaction of the outermost exposed charge transport layer, during electrophotographic imaging process in the field. Exposure to chemical species negatively affects the imaging member function. For example, exposure to the vapor amine species (from ammonia) emitted from common house cleaning agents have been seen to interact with the imaging member charge transport layer, causing material degradation to promote pre-mature onset of charge transport layer cracking and exacerbation of wear failure which severely cut short the functional life of the imaging member. In one particular instant, amine vapor impact on copy printout quality degradation has recently been seen when pre-printed papers (papers having pre-printed images which employed amine agents catalyzed ultraviolet (UV) cured ink) are used by customers for subsequent addition of xerographic images over the pre-printed paper blank spaces. Accumulation of amine residues deposition onto the imaging member charge transport layer surface, after repeatedly making contact with receiving papers during xerographic imaging process, is found to cause ghosting image defects print-out in the output copies. Since ghosting image defects in the output copies are unacceptable print quality failures, so it does require frequent costly imaging member replacement in the field. With all these issues and failures described above, therefore there is an urgent need to resolve these issues and extend the service life of the imaging member in the field. In particular, by the formulation of a charge transport layer that is resistive to amine specific effect to resolve the current pre-printed paper ghosting image defects print out problem.                Relevant prior arts to the present disclosure include U.S. Patent Publication No. 20090253060; U.S. Patent Publication No. 20090253058; U.S. patent application Ser. No. 13/034,654; and U.S. patent application Ser. No. 12/828,138.        
In addition, U.S. Pat. No. 7,504,187 discloses embodiments of electrophotographic imaging members, such as layered photoreceptor structures and process for making and using the same. In particular, the embodiments pertain to an improved electrophotographic imaging member having a protective overcoat layer comprising a low surface energy polymeric material to enhance the imaging member physical/mechanical function as well as render its service life extension and a process for making and using the member.
The above disclosures show that, while attempts to resolve charge transport layer failures described above have been successful with providing a solution, often times the success is negated due to the creation of another set of problems. Therefore, there is an need to provide improved imaging members that have robust outer layer to effect mechanical function and render resistivity to contaminant exposure/chemical attack for service life extension, without causing the introduction of other undesirable problems.